Amplifier



y 1- w. s. PERCIVAL 2,240,715

AMPLIFIER Filed Oct. 21. 1937 INVENTOR W/Zl l4 s izlymclm Mil-W ATTORNEY Patented May 6, 1941 AMPLIFIER William Spencer Percival, Ealing, London, Eng- Limited, Great Britain land, assig' r to Electric & Musical Industries Middlesex, England,

a company of Application October 21, 1937,.Serial No. 170,140 In Great Britain October 28, 1936 Claims. (c1. 179-171) This invention relates to amplifying circuits comprising electron dischargevalves.

It is known that in amplifying circuits the anode-to-grid capacity of the triode valve can cause undesirable coupling between the tuned input and anode circuits and it is well known that such coupling can be avoided byutilizing the neutrodyne principle or by substituting for the triode a screen grid valve.

It is also known that the grid-to-cathode capacity of the valve seriously limits the amplification of high frequencies when resistance capacity coupling is employed and also limits the dynamic impedance of a tuned circuit at ultra high frequencies. With a view to reducing the effective value of the input capacity of a thermionic valve, it has been proposed to provide negative feedback by inserting an impedance element between cathode and earth. If the magnitude of the impedance element is large compared with the inverse of the mutual conductance of the valve, the potential of the cathode substantially follows the potential variations of the grid and hence the output from a circuit of this kind can be taken cuit impedance. Valves operating in this manner have been termed cathode follower valves and for further particulars concerning circuits including such valves reference should be made to the specification of U. S. Patent No. 2,178,985, issued November '7, 1939, to A. D. Blumlein.

Such circuits can advantageously be used in low-pass amplifiers for various purposes. The low input capacity presented by such a circuit is advantageous when it is desired to feed signals thereto from a high impedance source without loss of the high frequencies and, in addition, the low output impedance is eminently suitable when it is desired to feed signals into a device of high effective shunt capacity, such as va triode amplifying valve, without attenuation of the high frequencies.

If, however, it is desired to employ cathode follower circuits in band pass circuits as by tuning the input and output circuits, then it is found that undesired coupling occurs between the input and output circuits owing to the grid-cathode capacity which at high frequencies of, for example, 45 megacycles may present a lower impedance than either the input or output circuits. In addition, the phase relationships in the input and output circuits is such that uncontrolled oscillation is liable to occur.

It is the chief object of the to provide an improved circuit with a present invention View to from across the cathode cirovercoming the above-mentioned disadvantages which arise due to coupling between input and output circuits. q

According to the invention, an amplifying circuit is provided, comprising an electron discharge valve having input and output circuits tuned to the same frequency and wherein the inherent capacity of such valve, which is effective in introducing coupling between said input and output circuits, is tunedby the addition of an inductance to resonate at the frequency to which the said input and output circuits are resonant.

In the first mentioned case the inherent capacity which is effective to cause coupling is the anode-to-grid capacity of the valve and in the second case the inherent capacity is that between grid and cathode. Since the anode-to-grid capacity in the one case and thegrid-to-cathode capacity in the other case is tuned to the frequency to which the input and output circuits are resonant, the tuned circuit thus formed presents a-high impedance at its resonant frequency so that the inter-electrode capacity is thus rendered substantially ineffective in respect of feedback thus preventing detrimental self oscillation. If the inter-electrode capacity to be tuned is small for example'as in the case of the grid-cathode capacity where the valve of the circuit is arranged to operate as a cathode follower, the inter-electrode capacity or capacities of the valve may be tuned in conjunction with additional capacities.

The method of carrying the invention into effect will be fully understood from the following description with reference to the drawing accompanying my invention, in which:

Fig. 1 illustrates a circuit in accordance with the invention and employing a valve arranged to function as a cathode follower.

Fig. 2 is a circuit similar to that shown in Fig. 1 but coupledto a further amplifying valve and,

Fig. 3 illustrates a circuit employing two valves functioning as cathode followers in cascade.

As shown in Fig. 1 of the drawing, the reference numeral 4 indicates a triode valve to which signals to be amplified are applied between grid and earth or the negative terminal of a suiable source of high tension via the secondary winding 5 of an input transformer, the winding 5 being tuned in conjunction with theefiective input capacity which is chiefly the grid-toanode capacity, to resonate at a desired frequency. The anode of the valve is connected through a load resistance 6 to the positive terminal of the high tension source a decoupling condenser 1 being associated with the anode as shown. The valve is arranged to function as a cathode follower for which purpose an impedance element 8 is connected between cathode and the negative terminal of the high tension supply, the impedance element being constituted by an initself, constitutes the tuned output circuit. The cathode is biased by a by-pass condenser Ill.

With the circuit so far described, it will be found that undesired coupling occurs between the input and output circuits owing to the feedback which occurs via the grid-cathode capacity. In

accordance with the invention, this is avoided by tuning the grid-to-cathode capacity with an inductance ll arranged in parallel with the gridcathode capacity so that the tuned circuit thus formed is resonant at the frequency to which the input and output circuits are tuned. The valve circuit thus becomes substantially purelyresistive in respect of the common frequency to which its input and output circuits are tuned. It will be appreciated that the capacity of condenser in is so large that this element may be neglected as far as the tuning is concerned.

It may be found that in some cases the small grid-cathode capacity necessitates an unduly large inductance, in which case the self-capacity of the valve may be augmented by the addition of a suitable condenser. In addition, the inductances 5 and 8 may also be tuned in conjunction with additional condensers.

Fig. 2 illustrates a modification of the circuit shown in Fig. 1, in which the output from the cathode of the valve is applied via a step-up autotrans'iormer to the grid of a succeeding valve. As shown in Fig. 2, an auto-transformer I2 is connected between grid and cathode of a pentode valve 13, impedance between the tapping point and the lower end of the coil 52 constituting the cathode circuit impedance of the valve 4, whilst the whole of the coil serves to apply stepped-up voltages to the grid of the valve 13. The cathode in this valve is biased by a resistance i4 shunted by a by-pass condenser It.

It is possible to employ. two or cathode follower circuits in cascade similar to that shown in Fig. 1, the low output impedance of one stage working, for example, via a step-up transformer into the high input impedance of the succeeding stage.

When using a triode valve with the anode effectively earthed the feedback method described in the specification of application U. S. Serial No. 38,631, filed August 30, 1935, of A. D. Blumlein, which is employed in the above described cases only reduces that part of the input capacity which is due to the grid-cathode capacity. The eifect of the grid-anode capacity remains and forms the input capacity. If it is desired to reduce still further the input capacity a screen grid valve may be employed with the screen decoupled to the cathode. The feedback action is then effective to reduce the grid-screen capacity. The resultant input capacity is in this case very much reduced compared with the triode case.

A circuit designed for passing a very wide freare coupled through a step-up auto-transformer .I8. The screening grids of the valves l6 and I! more stages of are decoupled by condensers I 9 and 20 connected between the respective screening grids and the cathodes. Each of the valves is arranged to a cathode follower, for which purpose the cathodes of the valves each have an impedance element in the form of resistances 2| and 22 and each valve is biased by resistances 23 and 24, shunted by by-pass condensers 25 and 26. The grid-to-cathode capacity of each valve is, in accordance with the invention, tuned by the insertion of inductances 21 and 28, as shown.

Since the input capacities of the valves are now very small the circuit is especially suitable for use at very high frequencies at which very small tuning capacities are usually desirable.

The resistance 2| is large compared with the inverse of the mutual conductance of the valve l6, and is coupled to the transformer l8 through frequency. By suitably adjusting the leakage of i8 and the position of the tapping point thereon, the required passband and the greatest gain consistent therewith can be obtained. A circuit employing the leakage inductance for this purpose is described in U. S. Patent No. 2,280,519, issued May 14, 1940, to W. S. Percival. If the leakage inductance of i8 is not sufficient, then an additional inductance may be inserted in series with the condenser 29.

Although in the specific description reference is made to valves operating as cathode followers with the output taken from the cathode, it is to be understood that the invention is not to be limited thereto. For example, in some cases it may be desirable to reduce the input impedance by employing a cathode follower, Whilst at the same time obtaining amplification, in which case the output from the circuit may be taken from If a triode valve is used in such an arrangement, oscillation can be prevented in accordance with the invention by tuning the anode-to-grid capacity. The invention can also be applied to other types of amplifiers in which the output is taken from a tuned anode circuit.

Having described my invention, what I claim 1s:

1. An amplifying system comprising an electron discharge vaive having a cathode, a control electrode, and an anode, a source of voltage having positive and negative terminals, a parallelly connected resistor and condenser, an impedance connected between the negative terminal of the voltage source and one junction point of the parallelly connected resistance and condenser, the other junction point of said parallelly connected resistor and condenser being connected with a cathode to said valve, an input circuit connected between the control electrode and said negative terminal, said input circuit being tuned to a predetermined frequency by the effective in said valve, an auxlliary inductance first named junction point, said inductance being of such magnitude as to be tuned to said predetermined frequency by the control electrode-cathode capacity of said valve, a load circult, and means to supply output energy from across said impedance to said load circuit.

2. An amplifying system as claimed in claim 1 wherein the impedance is tuned to the same fre quency as the input circuit is tuned.

3. {in amplifying system as claimed in claim 1 wherein the impedance is a resistance whose value 5. An amplifying system as claimed in claim 1 and wherein the impedance comprises one winding of an auto-transformer and comprising in addition a self-biased electron discharge device having a control electrode and an anode and cathode, and connections coupling the autotransformer to the control electrode-cathode circuit of said device.

WILLIAM SPENCER PERC'IVAL. 

